A deposit to be mined by underground methods can be accessed by a number of
methods:
• Adit
• Decline or ramp
• Inclined shaft
• Vertical shaft.
Adits are an economical approach when the orebody is above the general floor
elevation i.e. suitable in hilly or mountainous terrain. Incline shafts are limited to
relatively shallow deposits, and because they are developed on an incline,
development lengths for a given depth are the three to five times longer than for a
vertical shaft. Vertical shafts are the preferred method for deposits deeper than 300
m but the development rate is slow and construction costs are very high. Declines or
ramps offer early access to shallow deposits, which develops the ore body
expediently, but are generally developed at a gradient of approximately 12 per cent.
Decline haulages have become an attractive alternative to shaft hoisting, and over
recent years the role of decline access has become more widespread throughout
South Africa. Traditionally, South Africa has enjoyed the use of shaft systems, largely
due to the large knowledge base of mining the Witwatersrand Basin, where vertical
and inclined shafts were the norm. South Africa has also had the advantage of cheap
electricity, giving shafts a definite economic advantage. However, in recent years
the national power utility ESKOM has undergone an expansion programme that has
led to tariff increases of nearly 100% over a three-year period.
Based on the changes in electricity tariffs and technological improvements to
underground haulage trucks, the economic inputs to access development have
changed. This paper reviews mine access for shallow deposits as currently applied in
South Africa. Based on current economic inputs, the paper investigates at what
point a vertical shaft would be more economical than a decline system utilizing
typical South African mining equipment.